1. Field of The Invention
This invention generally relates to a highly-efficient encoding technique to be employed for efficiently performing a coding (or encoding) in digital-signal recording, transmission and display apparatuses to generate a smaller amount of codes (hereunder sometimes referred to simply as code amount). More particularly, this invention relates to a coding apparatus adapted to perform a coding of a moving picture signal by controlling a transfer rate and also relates to a recording medium on which a code generated by the coding apparatus is recorded. Further, this invention relates to a reproducing apparatus for reading moving picture information from the recording medium. More particularly, this invention relates to a variable transfer rate recording information reproducing apparatus for reproducing moving picture information recorded on a recording medium, which information represents a code generated by changing an amount of information corresponding to a predetermined period of time, and also relates to the recording medium on which the moving picture information is recorded.
2. Description of The Related Art
When a variable length code is used in performing a highly efficient coding of an image signal, a part of an image represented by the image signal, which part contains a small amount of image information, is coded or converted into a small amount of codes. Therefore, it is reasonable for increasing the efficiency of coding to employ a variable length coding. Especially, in case of an inter-image predictive coding of each frame or field of a moving picture, an amount of codes corresponding to a part having no motion is very little. Thus, the efficiency of transmission or recording of information or codes can be increased if the transmission or recording is effected by making the visual picture quality of a moving picture nearly constant and changing a transfer rate according to an aspect of the moving picture.
Such a way of transmitting information is called an asynchronous transmission mode (ATM) which is extensively studied as a packet transmission in the field of communication. In this case, basically, codes are generated at a predetermined transfer rate. Further, packets (or cells) are discarded on a transmission network if necessary. Thus, an amount of information or codes is controlled.
Meanwhile, in case of recording media termed package media (for example, a video tape recorder (VTR), a video disk or the like as presently used), information is recorded thereon at predetermined tape speed or a predetermined rotational speed. As the result, an amount of information recorded on the recording medium in a unit period of time (or a unit time slot) becomes constant. Therefore, the capacity of the recording medium can be indicated in terms of the length of available recording time. For instance, in case that a moving picture of 100 minutes is recorded on a recording medium whose maximum recording time is 120 minutes, the remaining capacity of the recording medium corresponding to the recording time of 20 minutes is space for recording another picture.
To make the most of the capacity of a recording medium, a transfer rate should be changed according to the contents of a picture to be recorded and thus the total amount of codes recorded thereon should be suited to the capacity thereof. However, in case of a conventional coding apparatus, the transfer rate is fixed to a constant value as will be described hereinbelow by referring to FIG. 1.
FIG. 1 is a schematic block diagram for illustrating the configuration of an example of the conventional coding apparatus. Incidentally, the coding apparatus of FIG. 1 is in conformity with CCITT (International Telegraph and Telephone Consultative Committee) specified standard system.
As shown in this figure, image signals are inputted to a predictive subtracter 2 and an activity detector 4 through an image input terminal 31. Incidentally, what is called activity indicates a degree of change in data, which is represented by an image signal, between adjacent blocks. In the predictive subtracter 2, an inter-frame prediction signal (hereunder sometimes referred to simply as a prediction signal) inputted from an inter-image predicting device 13 is subtracted from the image signal. Then, a prediction residual signal representing a result of the subtraction is outputted from the predictive subtractor 2 to a discrete cosine transform (DCT) device 3.
Next, in the DCT device 3, a DCT is performed on the prediction residual signal. Then, a signal representing a result of the DCT is outputted from the DCT device 3 to a quantizer 6 whereupon a quantization is effected according to a quantization step width (namely, a quantization step size) designated by information inputted from a quantization controller 5 thereon.
The quantized signal is next inputted to a variable length coder 7 and a local decoder 15. Further, in the device 7, information represented by the inputted signal is converted into a compressed code. Subsequently, a signal representing the compressed code is inputted to a buffer 32. Note that data (namely, the compressed code) inputted to the buffer 32 is converted by the device 7 into a variable length code. Thus, an amount of generated codes changes or fluctuates constantly. However, such change in amount of the generated codes is absorbed by the buffer 32. As the result, the generated code is outputted to a decoding apparatus (shown in FIG. 3) from a code output terminal 11 at a constant transfer rate.
On the other hand, in the local decoder 15, an inverse quantization and an inverse DCT are performed on the quantized signal inputted thereto. Thereby, the code is decoded and as a consequence, a reproduced prediction residual signal is produced. Then, the reproduced prediction residual signal is inputted to an adder 14 whereupon the inter-frame prediction signal inputted from the inter-image predicting device 13 is added to the reproduced prediction residual signal inputted from the local decoder 15. Subsequently, a reproduced image signal representing a result of this addition is supplied to an inter-image predicting device 13 whereupon the reproduced image signal is delayed by one frame and a motion compensation is effected to generate an inter-frame prediction signal. The generated inter-frame signal is supplied from the inter-image predicting device 13 to the prediction subtracter 2 and the adder 14.
Further, the quantization is controlled according to what is called an occupancy of the buffer and to the activity of an original image.
Thus, information representing the occupancy of the buffer 32 and the activity detected by the activity detecting device 4 is inputted to the quantization controller 5 whereupon a quantization step width is set according to the inputted information. Further, a signal indicating the quantization step width is inputted to the quantizer 6. Incidentally, the quantization step width is set in such a manner to be wide (namely, quantization is coarse) when many codes are stored in the buffer 32 and to be narrow (namely, quantization is fine) when the buffer 32 is almost empty. Such a characteristic of the quantization step width is shown in FIG. 2.
FIG. 2 is a diagram for illustrating the manner of the quantization control, which shows an example of the relation between the occupancy of the buffer and the quantization step width.
In case of controlling the quantization according to the activity, the quantization step width is wide (namely, the quantization is coarse) when the activity is large. Further, the quantization step width is narrow (namely, the quantization is fine).
This is due to the fact that in case of a block having large activity, change in image signal is large and that an error is not noticeable and that in contrast, in case of a block having small activity, even a small error is conspicuous.
Practically, the value of the quantization step width set according to the occupancy of the buffer is multiplied by a multiplier which ranges from (xc2xd) to 2 and is set according to the activity regarding each block.
At that time, if the average value of the activity is 1, the average value of the quantization step width does not change.
In case of this conventional coding apparatus, for instance, a variance of tone levels (or density levels) of pixels of a block is employed as an activity. Incidentally, when using DCT or the like, a quantization error is conspicuous in a block of an edge portion. Therefore, it is preferable that an edge portion is detected before the activity is determined, and subsequently, the activity of a block of the detected edge portion is regulated in such a way not to become large.
By using the activity of an input image as described above, subjective visual picture quality can be made to be uniform.
Next, a conventional decoding apparatus will be described hereinbelow by referring to FIG. 3.
FIG. 3 is a schematic block diagram for illustrating the configuration of an example of the conventional decoding apparatus.
As shown in this figure, codes inputted from a code input terminal 20 to a buffer 41 at a constant transfer rate are outputted to a variable length decoder 22 in synchronization with a processing to be effected therein.
In he variable length decoder 22, the variable length code is converted into a fixed length code. Then, in the dequantizer 23, the fixed length code is converted into a quantization representative value (hereunder sometimes referred to simply as a quantization representative). The quantization representative is inputted to an inverse DCT device 24 whereupon an inverse DCT is performed on the quantization representative to produce a reproduction predictive residual signal. Then, the reproduction predictive residual signal is applied to the adder 14.
Next, in the adder 14, a prediction signal inputted from the inter-image predictor 13 is added to the reproduction predictive residual signal to generate a signal (hereunder referred to as a reproduced image signal) representing a reproduced image. This reproduced image signal is outputted from an image output terminal 25. Further, the reproduced image signal is also inputted to the inter-image predictor 13.
However, as described above, in case of the conventional coding apparatus, the transfer rate is controlled in such a manner to be constant. Thus, the conventional coding apparatus has a drawback in that although an amount of codes corresponding to a portion of an image is sufficient and a corresponding quantization step width is too narrow, an amount of codes corresponding to another portion of the image is insufficient and a corresponding quantization step width is large (namely, quantization is coarse) and thus the picture quality is degraded.
Further, in case of the conventional coding apparatus, a coding is performed at a constant transfer rate irrespective of the length (or duration) of a moving picture. Thus, the conventional coding apparatus has another drawback in that if the length of a moving picture is shorter than the maximum recordable time of a recording medium to be used to record the moving picture, a part of the recordable area of the recording medium remains unused.
Meanwhile, in case of an apparatus for reproducing information from a recording medium as above described, it is usual that information representing an image or an audio is continuously read and reproduced from the medium and thus there is no necessity of special control of reproduction of the information.
In case of a constant-line-velocity (CLV) disk medium is which a line velocity is constant, a rotational frequency should be changed according to the position on a disk. However the rotational frequency dose  does not depend on the contents of information (for example, an amount of codes corresponding to information to be recorded for a predetermined period of time) but depend  depends on the position on the disk.
It is preferable for making the most of the capacity of a recording medium that the transfer rate for transferring image information to be recorded is changed according to the contents of a picture to be recorded (for instance, an amount of codes corresponding to a predetermined period of time) in such a manner to vary with portions of an image represented by the image information and that the image information is recorded as what is called variable transfer rate information and is reproduced from the recorded variable transfer rate information.
However, in case of the conventional coding apparatus, it is not possible that the relative speed between a recording medium and a reproducing head is changed frequency and quickly according to the transfer rate for transferring the variable transfer rate information. Thus, in case of the conventional coding apparatus, moving picture information is first recorded at a fixed transfer rate. Thereafter, the recorded moving picture information is continuously read as fixed transfer rate information. Therefore, the prior art has a drawback in that variable transfer rate information can not be reproduced from a recording medium on which the variable transfer rate information is recorded by changing the transfer rate each predetermined period of time.
The present invention is accomplished to eliminate the above described drawbacks of the prior art.
It is, therefore, an object of the present invention to provide a variable transfer rate control coding apparatus which and to a recording medium on which information generated by this variable transfer rate control coding apparatus is recorded.
Further, it is another object of the present invention to provide a variable transfer rate moving picture information reproducing apparatus which can reproduce variable transfer rate moving picture information (namely, moving picture information coded at a variable transfer rate changing every prescribed time period and recorded on a recording medium) from the recording medium at the time of decoding thereof without changing the relative speed between the recording medium and the reproducing head and to provide a recording medium suitable for the recording of the variable transfer rate moving picture information by the reproducing apparatus.
To achieve the foregoing object, in accordance with an aspect of the present invention, there is provided a variable transfer rate control coding apparatus which can distribute codes of an amount most suitable to the contents of a moving picture to be recorded on a recording medium and to a recording medium by obtaining an amount of codes to be temporarily generated to each unit period of time by a temporary or tentative coding, and then storing a value of the temporary amount of codes to be generated in each unit period of time (hereunder sometimes referred so as a temporary transfer rate), and setting a target value of an amount (hereunder sometimes referred to as a transfer rate) of codes to be sent in each unit period of time (namely, setting a target transfer rate) from a total of the temporary transfer rates and the recordable capacity of the recording medium and next performing an actual coding (hereunder sometimes referred to as a real coding) according to the target transfer rates. Moreover, there is provided a recording medium on which information is recorded by this variable transfer rate coding apparatus.
Namely, the variable transfer rate coding apparatus for performing a coding on a moving picture signal, which comprises temporary coding means for obtaining a temporary transfer rate representing an amount of codes to be generated in each unit period of time from the moving picture signal and target transfer rate setting means for setting a target transfer rate corresponding to each unit period of time from the temporary transfer rate in such a manner that a total amount of codes generated from the moving picture signal is equal to a predetermined value. Thus, a coding of the moving picture signal is effected by controlling an amount of generated codes in accordance with the target transfer rate corresponding to each unit period of time.
Further, the target rate setting means of a preferred embodiment (namely, a variable transfer rate control coding apparatus) of the present invention is provided with a transfer rate converting device which converts the temporary transfer rate corresponding to each unit period of time into the target transfer rate corresponding to each unit period of time by making increase in the target transfer rate be less than increase in the corresponding temporary transfer rate and limiting the maximum value of the target transfer rate to a predetermined constant value.
Furthermore, in a preferred embodiment (namely, a recording medium) of the present invention, the transfer rate of the moving picture information recorded thereon is not constant, and an amount of codes recorded thereon and generated from the entire moving picture is made to be nearly equal to the available (or recordable) capacity of the recording medium.
Thus, as described above, in case of the variable transfer rate control coding apparatus, an amount of codes to be temporarily generated in each unit period of time is obtained by a temporary coding, and thereafter a target value of an amount (hereunder sometimes referred to as a transfer rate) of codes to be sent in each unit period of time (namely, setting a target transfer rate) is set from a total of the temporary transfer rates and the recordable capacity of the recording medium and finally, an actual coding is performed according to the target transfer rate. As the result, an amount of codes corresponding to a portion of an image, the quantization of which would be excessively fine in case of the conventional coding apparatus, becomes small and an amount of codes corresponding to another portion of an image, the picture quality of which would be degraded by coarse quantization in case of the conventional coding apparatus, becomes large.
Thus, in accordance with the present invention, amounts of codes are suitably distributed to portions of the image according to the contents of information representing the portions, and moreover the picture quality is improved. Furthermore, the target transfer rate corresponding to each unit period of time is set in such a manner that a total amount of codes is constant. Therefore, by controlling a coding according to the target transfer rate, even if the length or duration of a moving picture is changed, all codes are recorded on the recording medium without waste thereof. Further, in accordance with the present invention, fluctuation in quantized value becomes small and as a consequence, change in picture quality due to the control of a coding becomes also small.
Especially, regarding an inter-image prediction coding, an amount of data generated corresponding to a portion, in which there is no motion, of a picture is small. Consequently, the picture quality is improved sharply.
Further, although the transfer rate may vary with unit periods of time, the transfer rate is unchanged within each unit period of time. Thus, the variable transfer rate control coding and decoding apparatuses of the present invention have good compatibilities with fixed transfer rate coding and decoding apparatuses, respectively. Further, the formers can be realized by adding systems each for controlling a total amount of codes to the latter, respectively.
Moreover, if the variable transfer rate control coding apparatus of the present invention and the conventional coding apparatus use a recording medium of same recording capacity and obtain same picture quality, the variable transfer rate control coding apparatus of the present invention can record longer than the conventional coding apparatus does.
Furthermore, the recording medium of the present invention is a medium on which image information generated by the variable transfer rate control coding apparatus of the present invention is recorded. Each time when a unit period of time passes, the transfer rate is controlled or regulated according to the contents of a portion, which corresponds to a unit period of time, of information to be recorded, the total amount of codes obtained from the information and the recording capacity of the recording medium. Thus, information is recorded on the full recording capacity of the recording medium. Consequently, the picture quality of a moving picture reproduced from the recording medium is much better than that of a moving picture reproduced from codes recorded by the conventional coding apparatus employing a fixed transfer rate.
As described above, the variable transfer rate control coding apparatus and the recording medium of the present invention have excellent effects in practical use.
Further, in accordance with another aspect of the present invention, there is provided a variable transfer rate information recording medium, on which moving picture information coded by changing the transfer rate is recorded and on which reading operation control information is also recorded in a multiplex recording manner. The reading operation control information is used for directing information reading means of a reproducing apparatus to read information or to stand by instead of reading the information, when reproducing information from this recording medium.
Moreover, in accordance with a further aspect of the present invention, there is provided a variable transfer rate information recording medium, on which moving picture information coded by changing the transfer rate is recorded and on which information representing an amount of codes generated from a portion corresponding to a unit period of time of a moving picture is also recorded in a multiplex recording manner.
Furthermore, in accordance with still another aspect of the present invention, there is provided a variable transfer rate information reproducing apparatus for reproducing moving picture information coded while changing a transfer rate every predetermined period of time and recorded on a recording medium, which apparatus is provided with information reading means for intermittently reading information according to an amount of information necessary for decoding each predetermined period of time. When information should not be read, the information reading means ceases to read information and stands by without changing the relative speed between a reproducing head and the recording medium. Further, in case that it is not necessary to read information, the information reading means reads the read information again and thereafter discards unnecessary information.
Moreover, in accordance with yet another aspect of the present invention, there is provided a variable transfer rate information reproducing apparatus for reproducing moving picture information coded while changing a transfer rate every predetermined period of time and recorded on a recording medium, which apparatus is provided with a decoding buffer for temporarily storing read information, buffer means for outputting an occupancy of the decoding buffer, reading controller for outputting reading control information according to the occupancy of the decoding buffer, and information reading mean which occasionally does not read information but stands by according to the reading control information.
Furthermore, in accordance with another aspect of the present invention, there is provided a variable transfer rate information reproducing apparatus for reproducing information from a recording medium on which reading operation control information for controlling the information reading means to read information or stands by instead of reading information and moving picture information are recorded in a multiplex recording manner, which apparatus is provided with control information separation means for detecting the reading operation control information, and in which information reading means occasionally does not read information but stands by according to the reading operation control information.
Further, in accordance with a further aspect of the present invention, there is provided a variable transfer rate information reproducing apparatus for reproducing information from a recording medium on which information representing a transfer rate of a moving picture signal corresponding to each unit period of time and moving picture information coded while changing the transfer rate thereof every unit period of time are recorded in a multiplex recording manner, which apparatus is provided with transfer rate information separation means for detecting code amount information representing a transfer rate of the moving picture signal corresponding to each unit period of time, reading operation controller for outputting control information according to the code amount information and information reading means which occasionally does not read but stands by according to the reading operation control information.
Additionally, in accordance with a still further aspect of the present invention, there is provided a variable transfer rate information reproducing apparatus for reproducing moving picture information from a recording medium on which the moving picture information coded while changing a transfer rate every unit period of time is recorded as a packet of information, which apparatus is provided with packet code amount detector for detecting an amount of the packet of read information, reading operation controller for outputting reading operation control information according to an amount of information corresponding to the packet and information reading means which ceases to read information and stands by according to the reading operation control information.
Further, in accordance with a yet further aspect of the present invention, there is provided a variable transfer rate information reproducing apparatus for reproducing information on a recording medium on which fixed-transfer-rate information coded while fixing a transfer rate and moving picture information coded while changing a transfer rate every predetermined period of time are recorded in a multiplex recording manner, which apparatus is provided with a fixed-transfer-rate detector for detecting an amount of codes generated from the information for the period of the predetermined length, information reading means for reading information recorded on the recording means and reading operation control means for controlling the information reading means in such a manner that the amount of codes generated in each period of the predetermined length becomes constant. The information reading means occasionally does not read information and stands by according to the reading control information.
As described above, on the recording medium of the present invention, what is called variable transfer rate information obtained by controlling a transfer rate every period of the predetermined length is recorded. Further, reading control information for controlling the reproducing apparatus to read information or stands by instead of reading information and information indicating an amount of codes representing the variable transfer rate information are also recorded thereon in a multiplex recording manner.
In case of the variable transfer rate information reproducing apparatus of the present invention, the variable transfer rate information recorded on the recording medium is ready by the information reading means provided in the reproducing apparatus.
Incidentally, information is read by the information reading means from the recording medium at a predetermined rate (namely, information of a predetermined amount is read in a period of predetermined length).
The read information is stored in the decoding buffer. Thereafter, information of an amount necessary for decoding is outputted from this buffer. In case of decoding codes generated from fixed-transfer-rate information, the amount necessary for decoding is constant amount corresponding to a period of the predetermined length. In contrast, in case of decoding codes generated from variable transfer rate information, the amount necessary for decoding varies every period of the predetermined length.
Further, an amount of information stored in the buffer, as well as the occupancy of the buffer, changes according not only to the decoding rate (or the reading rate) but to whether information is read from the recording medium (or whether a reading operation is ceased and the apparatus stands by).
Moreover, the information reading means is controlled to read information or cease to read and stand by according to the occupancy of the buffer and the control information recorded on the recording medium in a multiplex recording manner.
As described above, the variable transfer rate information reproducing apparatus of the present invention is provided with the information reading means for intermittently reading information according to the amount of information necessary for decoding codes generated from the moving picture every period of the predetermined length. Further, this information reading means is controlled according to the occupancy of the buffer and the amount of codes generated from information in a period of the predetermined length to read information from the recording medium or to stand by without reading information. Thereby, the information coded at a variable transfer rate changing every predetermined period of time and recorded on the recording medium can be reproduced without changing the relative speed between the recording medium and the reproducing head.
Moreover, in case of the reproducing apparatus of the present invention, reading operation control information for controlling the information reading means and the information concerning the amount of codes generated in each period of the predetermined length are recorded on the recording medium in a multiplex manner and are thereafter used at the time of reproduction. This facilitates a control processing of the information reading means. Consequently, information recorded at variable transfer rate can be reproduced by utilizing a buffer of relatively small capacity and a small delay time.
As stated above, the variable transfer rate information reproducing apparatus and the recording medium used therein of the present invention have excellent effects in practical use.